The present invention relates to an electric power steering apparatus that employs an electric motor as a generating source of steering assistance force.
An electric power steering apparatus for a vehicle includes an electric motor for steering assistance and a reduction gear mechanism that transmits rotational force of the electric motor to a steering unit, so as to assist the motion of the steering unit with the rotation of the electric motor in response to rotation of a steering means, thereby alleviating the labor imposed on the driver for the steering operation.
The reduction gear mechanism is supported by a pair of bearings each provided on a respective side thereof in a housing, and includes a worm serving as a pinion that moves interlocked with the rotation of the electric motor, and a worm wheel serving as a gear wheel meshed with the worm.
The electric power steering apparatuses including such reduction gear mechanism include, as disclosed in Japanese Patent Application Laid-Open No. 2004-203154 for example, the one provided with a bent leaf spring, longer than the circumference of the bearing on the farther side from the motor and having an end portion warped radially of the bearing thus to be formed generally in an á-shape, so as to be fitted around the bearing inside the housing, so that the elastic restoring force of the end portion of the bent leaf spring biases the worm in a direction that shortens a distance between the center of rotation of the worm and the center of rotation of the worm wheel, to thereby reduce an amount of backlash at the meshing interface between the worm and the worm wheel and to thus minimize rattling noise due to the backlash caused when changing the steering direction.
The electric power steering apparatus can also be found in Japanese Patent Application Laid-Open No. 2001-233224. FIGS. 1A to 1C are diagrams for explaining an assembly process of the worm according to Japanese Patent Application Laid-Open No. 2001-233224. The electric power steering apparatus includes a retaining cylinder 105 having a retaining bore 104 concentric with a bearing 103 closer to an electric motor fitted inside the housing 100 and around a bearing 102 located opposite to the bearing 103 across a worm 101. Inside the retaining bore 104 in the retaining cylinder 105, a coil spring 106 is wound such that the axial center thereof forms a ring-shape, and retained concentrically with the retaining bore 104 (Ref. FIG. 1A), so that, when assembling a worm wheel 107, the meshing interface between the worm wheel 107 and the worm 101 biases the worm 101 shown in FIG. 1B in a direction that extends the distance between the centers of rotation (L), i.e. the direction indicated by an arrow Y in FIG. 1C, to thereby slightly warp a circumferential portion of the coil spring 106 between an inner and an outer circumferential surface thereof, so that the elastic restoring force of the coil spring 106 biases the worm 101 in a direction that shortens the distance between the centers of rotation L (Ref. FIG. 1C), thus minimizing the backlash amount at the meshing interface.